1. Field of the Invention
This invention relates to a method of manufacture of plastic film capacitors. More particularly, this invention relates to a process for heat treating plastic film capacitors of either the film-foil or metallized film type.
A plastic film capacitor ("PFC") is one in which plastic film is utilized as the dielectric of the capacitor. A simple plate PFC is comprised of a dielectric plastic film sandwiched between a pair of metal foil electrode plates. One common type of plastic film capacitor comprises a plate capacitor which is rolled up upon itself so as to form a cylinder. A second layer of dielectric material is added to prevent the two electrode plates from making physical contact and shorting out, and to effectively double the capacitance value of the capacitor. The ends of the rolled capacitor are coated with an appropriate conductive material to allow for the attaching of leads. A partially wound cylinder of this type is shown in FIG. 1.
An alternative PFC construction differs from the film-foil type in that each foil electrode plate is replaced with a metallization layer which is vapor deposited on the dielectric film, as shown in FIG. 2. Foil electrodes typically have a thickness on the order of 0.00015 inches to 0.00075 inches, while the thickness of a metallization layer ranges between 0.000001 inches and 0.000002 inches. By effectively eliminating the two thick layers of foil from the capacitor, a considerable size reduction can be realized. In addition, metallized film capacitors are more reliable than film-foil capacitors due to more consistent electrical properties.
The dielectric material of a film capacitor consists of a solid, non-conducting thermoplastic film. Many different types of dielectric film can be utilized; however, the primary ones in use at present include polyester, polypropylene, polycarbonate, polystyrene and polysulfone. The dielectric insulating material in a film capacitor functions to accurately maintain the two electrode plates at a minute separation distance. In addition, the dielectric strength (i.e., the capacity for handling voltage without electrically breaking down) is higher with a dielectric film capacitor than would be the case with a capacitor using air as the electrode separation medium. Therefore, a larger voltage gradient (voltage rating) can be applied to the capacitor. Also, a dielectric film capacitor has the ability to hold a greater charge than does an air or a vacuum capacitor.
During the film manufacturing process, longitudinal and/or lateral stress forces are applied to the film sheet so as to cause it to axially stretch. The stretching, which is also referred to as orientation, is typically done with the sheet heated and the deformation is locked into the film by rapidly cooling the material to a temperature below which there is insufficient energy available to allow the plastic to be mobile enough to revert to its original unstressed dimensions. After the orientation process, the film is metallized to form the electrode plates (if metallized rather than film-foil type capacitors are to be produced). In order to form a cylindrical capacitor, the metallized film layers (or film and foil combination) are spirally wound tightly around a mandrel to form a plurality of turns.
Once a capacitor has been wound with the oriented plastic film, it is subjected to a heat treatment at a temperature at which the film again becomes mobile and attempts to revert to the dimensions of its original unoriented state. This temperature is different for various films, and typically ranges between 60 degrees C. and 180 degrees C. For a biaxially oriented film the heat treatment causes the film layer to reduce in width and length dimensions and increase in thickness. This heat treating process is commonly referred to as heat shrinking or heat stabilization. The dimensional changes in the film cause the layers of the capacitor to physically bond together more intimately and increase the physical stability of the capacitor. The heat treatment forces any minute amounts of entrapped air out from between the various layers in the capacitor and causes the dielectric film to spirally pull down upon itself, thereby causing the winding to become more rigid. Had the film not been originally stretched and subsequently heat treated, the capacitor winding would be soft and full of air, thereby making it prone to various parameter changes as a result of slight environmental variations such as barometric pressure, humidity and temperature. By employing the heat treatment process, a capacitor having improved electrical characteristics and stability is obtained.
2. Description of the Prior Art
Heretofore, the heat shrinking process has been accomplished by placing the wound capacitors in a convection oven. Heating of the turns of the wound cylinders with this method is not accomplished uniformly, however, thereby resulting in several problems. When initially placed in a hot oven, the outer turns of the wound cylinder will reach shrink temperature before the inner turns since the dielectric film, like any plastic, has extremely poor thermal conduction characteristics. The outer turns will therefore begin to shrink before the remaining turns of the cylinder. The shrinking causes the thickness of the outer turns to increase. Since the inner turns have not yet reached shrink temperature (and therefore have not yet begun to shrink), the dimensional changes of the outer turns causes a great deal of radial pressure to be built up in the winding. This initial collapse of the outer turns is commonly referred to as thermal shock (FIG. 1, reference numeral 1). The radial pressure created by the thermal shock causes a minute amount of rippling in the inner turns of the dielectric film, causing air to be trapped in the cylinder rather than pushed out.
After the initial thermal shock, the inner turns begin to heat up. When the interior turns reach full heat shrink temperature they finally begin to shrink down, causing the mandrel to collapse slightly (FIG. 1, reference numeral 2). The shrinking of the inner turns causes the dielectric to be pulled toward the mandrel in a spiral fashion. Because of the radial pressure created by the thermal shock, the spiral pulling causes frictional strain on the film (FIG. 1, reference numeral 3). The longer the cylinder is heat shrunk, the greater is the reduction of the original oriented stress on the dielectric film. Reduction of the oriented stress tends to relieve the radial pressure caused by the thermal shock.
Although it is desirable to completely shrink the film and relieve all of the radial pressure built up due to the thermal shock, it is not possible to do so since the spiral frictional strain will cause degradation of the film past acceptable limits before full pressure relief can be accomplished (FIG. 1, reference numeral 3). This inability to fully relieve the radial pressure constitutes a significant disadvantage in the prior art method of manufacturing PFC's.
In addition to the problem of not being able to fully relieve the radial pressure caused by the thermal shock on the wound cylinder, heat shrink time for a convection oven shrunk winding is generally measured in terms of hours, as opposed to a period of minutes which are required to fully shrink a free sheet of film. If the turns of the winding heated up uniformly the heat shrinking procedure could be completed very quickly. Due to the thermal shock, however, the heat shrink time must be increased in order to relieve the radial pressure. This is required due to the fact that any residual radial pressure has an adverse effect upon the properties of the capacitor, despite the fact that the winding is mechanically rigid. These properties include drift (variation of capacitance over time), temperature coefficient, retrace (the ability of the electrical parameters of a capacitor to resume their initial values after environmental testing) and insulation resistance (the ratio of direct current voltage applied between terminals of a capacitor to the resulting leakage current, after the initial charging current has ceased).